This invention relates to a process of preparing diaryl ethers.
Diaryl ethers, such as diphenyl oxide, are useful as high temperature solvents and as components in heat transfer fluids and perfumes. In addition, diaryl ethers are intermediates in processes of preparing flame retardants and surfactants.
U.S. Pat. No. 4,092,364 discloses the production of diphenyl oxide by the caustic hydrolysis of chlorobenzene. Because the process operates at about 400.degree. C. and about 5000 psig pressure, an expensive alloy reactor is required. As a further disadvantage, several organic byproducts are formed which are not readily marketed. Even more disadvantageously, stoichiometric quantities of sodium chloride are produced as a byproduct which must be disposed.
U.S. Pat. No. 4,360,699 discloses a process of preparing diphenyl oxide comprising contacting phenol with aluminum. This reaction presumably proceeds by forming aluminum triphenate which then pyrolyzes to diphenyl ether and alumina. Disadvantageously, the reaction is stoichiometric in aluminum, requires high temperatures, and forms a solid aluminum oxide as a byproduct.
The most general method for making diaryl ethers is the Ullman condensation which allows for the preparation of a variety of symmetrical and unsymmetrical diaryl ethers. In this method, illustrated by the disclosure of H. Weingarten in the Journal of Organic Chemistry, 29, 1964, 977 and 3624, a metal phenate, such as potassium phenate, is reacted with a halobenzene, such as chlorobenzene, over a copper catalyst to yield the desired diaryl ether. As a disadvantage, this method produces stoichiometric quantities of metal halide as a byproduct which must be disposed.
It is also known to prepare diaryl ethers by the pyrolysis of diaryl carbonates, as disclosed in U.S. Pat. No. 4,596,680. Diaryl carbonates are typically manufactured by the reaction of phenols with phosgene. The latter reactant is undesirable for safety reasons. Moreover, when two equivalents of phenol react with one equivalent of phosgene, two equivalents of hydrogen chloride are produced as a byproduct. The hydrogen chloride must be neutralized with base which yields a waste salt stream which must be disposed.
It would be more desirable to prepare diaryl ethers simply by the dehydration of phenols. The dehydration was first reported by P. Sabatier and A. Mailhe in Compt. Rend., 151, 1910, 492-494, who passed phenol over a thoria catalyst at 410.degree. C. and atmospheric pressure to obtain diphenyl oxide and dibenzofuran as a byproduct. More recently, F. Claes and J. C. Jungers (Bull. Soc. Chim. Fr., 1962, 1042) evaluated the kinetics of phenol dehydration to diphenyl ether over thoria supported on pumice. They reported equilibrium data at temperatures from 400.degree. C. to 438.degree. C. Based on their data, it can be concluded that the reaction to form diphenyl ether is endothermic at temperatures below 460.degree. C. and exothermic at temperatures higher than that. At 400.degree. C. the equilibrium phenol conversion is only 37.0 weight percent. At 438.degree. C. the equilibrium phenol conversion is 57.1 weight percent.
In addition to the above, British Patent 911,246 discloses the dehydration of phenols to diphenyl oxides over thoria supported on alpha-alumina, and U.S. Pat. No. 4,898,982 discloses the dehydration of phenols to diphenyl oxides over thoria deposited on a neutral support, such as pure silica, pure zirconia, carbon or asbestos. Although the processes employing thoria provide good yields of diaryl ethers, the processes suffer from at least one serious disadvantage. Thoria is radioactive; thus, extra precautions are required to handle the catalyst, disposal options are limited, and government licensing is required.
There are very few examples of catalysts other than thoria which successfully dehydrate phenols to diaryl ethers. Japanese Patent Kokai No. 59-196835 discloses a process wherein a phenol is brought into contact with titanium oxide or zirconium oxide in the vapor phase to yield diaryl ethers. In the best example, a 37 percent conversion of phenol is achieved with 82 percent selectivity to diphenyl ether. Disadvantageously, the yield of diphenyl ether is only 30 percent at best.
Japanese Patent 63-72640 discloses a method of preparing diphenyl ethers in which phenol and/or a substituted phenol is brought into contact in the liquid phase with a catalyst containing a crystalline metal silicate whose molar ratio of silicon oxide to trivalent metal oxide, SiO.sub.2 /M.sub.2 O.sub.3, is at least 12, and preferably, in the range from 40 to 3,000. "M" may be aluminum. The metal silicates disclosed by this patent include crystalline metal silicates having a pentasil structure or zeolites with a ZSM-5 structure. It is taught that after 20 hours at 300.degree. C. the phenol conversion is 43 mole percent and the selectivity to the ether is 90 mole percent. Disadvantageously, this process requires pre-pressurization to high pressures.
U.S. Pat. No. 4,536,485 broadly disclose the preparation of alkyl and aryl ethers from aliphatic and aromatic alcohols. Synthetic zeolites, specifically ZSM-5 and zeolite Y, are taught to be useful catalysts for the process. A method of treating the catalyst is taught to improve catalytic lifetime.
In view of the above, it is clear that a need exists to find a method of producing diaryl ethers which is selective and efficient and which minimizes waste formation and does not use phosgene or radioactive materials.